The present invention relates to reinforcing apparatus for a weld seam and particularly relates to a compact, remotely installable weld reinforcement device for maintaining structural integrity of welded members in the event of weld failure, for example, in a nuclear reactor.
A weld seam is typically formed between adjacent abutting welded members. For example, in a boiling water nuclear reactor pressure vessel, a cylindrical shroud is used to direct water flow through the fuel core. The bottom of the shroud is attached to a shroud support structure which is welded to the bottom of the pressure vessel. The shroud support structure consists of a baffle plate, cylinder and support legs. The baffle plate and cylinder separate the water flow through the annulus between the pressure vessel and shroud and the inside of the shroud. The shroud is a structural component used to support the core plate, top guide grid and shroud head/separator. The shroud consists of several stacked cylindrical sections typically constructed by welding together formed or rolled plates. Conventionally, each cylindrical section is made from two or three plates. The cylindrical sections are also of varying diameters. The transition between the different diameter cylinders is constructed by welding a circular ledge, e.g., a flat ring, between the ends of the cylinder sections. Thus, the shroud contains multiple vertical and circumferential welds.
The core plate which is bolted to the middle section of the shroud is used to laterally support the lower end of the fuel assemblies and the upper end of control rod guide tubes. The core plate also functions as a baffle plate to direct core flow through the fuel assemblies. The directing of the core flow creates a differential pressure across the core plate. Thus, the core plate is designed to support vertical and horizontal loads which, in turn, are transferred to a shroud. Also bolted to the upper section of the shroud is the shroud head/separator. The shroud head/separator is used to remove moisture from the steam created within the core. The moisture separation causes a pressure drop through the shroud head/separator and thus creates an upward load which is transferred to the shroud. During operating conditions and/or seismic events, both horizontal and vertical loads must be transferred through the shroud and the multiple welds which are used to construct the shroud. Failure of a weld can result in loss of integrity of the shroud, proper support for the core/core internals and may require shutdown of the nuclear plant.
Shroud repair hardware to maintain the integrity of the shroud in the event of failure of the welds has been designed in the past. For example, brackets have been bolted to the outside of the shroud to bridge across the circumferential weld joints. The brackets were installed on the outside of the shroud in the annulus. The installation required that for each bracket four holes be machined through the shroud. This repair was designed to provide an alternate load path to circumferential weld joints. This type of apparatus, however, limited access into the annulus between the core shroud and the pressure vessel.
Another example of repair hardware involved use of multiple tie rods which connect the upper end of the shroud to the shroud support. The material used for the tie rods had a lower coefficient of thermal expansion than that of the shroud material. Bumpers of various geometries had been used to provide lateral support between the pressure vessel and the outside of the shroud. Typically, a bumper is placed at the elevation of the top guide grid, core plate and shroud to shroud support joint. This method, as in the prior method, likewise limited access into the annular region. Some designs also required removal of the repair hardware to enable access to the jet pump inlet mixers and also limited access for inspection of the pressure vessel from the inside. Accordingly, there has developed a need to provide an alternate load path across a weld joint, either circumferential and/or vertical, in a shroud weld to carry tensile and shear loads in the event of weld failure and which would meet original design requirements placed on the shroud without change in outline geometry of the shroud and annulus to maintain access, while simultaneously affording remote installation with minimum size and mass of the repair and minimum installation time.
In accordance with a preferred embodiment of the present invention, a pocket, preferably in the shape of an I-beam or dogbone, is formed, e.g., machined into the adjoining surfaces of the welded members to a predetermined depth, with the reduced intermediate section of the pocket straddling the weld seam. The pocket may be located on the inside or outside of the welded members, e.g., the members of the shroud of a nuclear reactor. The edges of the pocket may be straight or slightly tapered. The edges adjacent and parallel to the weld seam, however, are preferably tapered. A projection, for example, a cylindrical section within the pocket at one or both ends of the pocket, is provided, e.g., left unmachined and hence projects from the base of the pocket toward the surface of the members.
A generally complementary-shaped, e.g., I-beam or dogbone-shaped, bracket or element is provided and placed in the pocket. The edges of the element adjacent to and parallel to the weld seam are tapered complementary to the taper of the edges of the pocket. This taper allows for a zero clearance fit-up between the pocket and the element along an axis perpendicular to the weld seam. A small clearance between the element and the pocket is provided along an axis parallel to the weld seam to facilitate installation of the bracket. The element also has a clearance hole aligned with the projection in the pocket.
To retain the element in the pocket, a retaining clamp is installed into the hole in the element and around the projection left in the pocket. The retaining clamp may comprise a wedge overlying a sleeve and a driver, for example, a nut, screw threaded to the sleeve and disposed in the annular opening about the projection. By threading the nut and sleeve, the clamp wedge expands between the bore of the hole and the outside of the projection due to the tapered design of the wedge and sleeve. The retaining clamp is required to secure the element during non-operating/startup conditions. An ancillary locking feature may be inmcluded as part of the retaining clamp, if desired.
Preferably, the coefficient of thermal expansion of the material of the element is lower than the coefficient of expansion of the material of the shroud, i.e., the joined members. Thus, during heatup/operation of the plant, the element will be placed in tension and the shroud weld joint in compression. Thus, the load path perpendicular to the weld seam is carried through the element as a tensile load, while the load applied parallel to the weld seam is transferred as a shear load through the cross-section of the element.
In a further embodiment of the present invention, the enlarged pocket portions at opposite ends of the pocket may have holes through to the opposite surface of the joined members. Bolts may be installed from the opposite side of the members and received through openings of the element. Nuts may be applied to the bolts to retain the elements in the pockets. In this embodiment, loads applied normal to the plane of the shroud surface may be transferred through the element and bolt assemblies. This design is particularly useful where access to both sides of the shroud is available.
A further option is to apply a cladding or plate over the element and shroud. The cladding or plate would eliminate the need for a clamp retainer or bolt assemblies.
It will be appreciated that the size, material and quantity of the elements and clamp retainers or bolts/nuts required for each weld seam is based on the required load that must be transmitted by the weld seam. Also, the outside contour of the element may be flat or curved to match the radius of the shroud.
In a preferred embodiment according to the present invention, there is provided a weld reinforcement comprising first and second members welded to one another forming an elongated weld seam, a continuous pocket formed in the members opening through corresponding surfaces thereof and defining first and second pocket portions on opposite sides of the weld seam, the first pocket portion adjacent one end of the pocket having a laterally enlarged recess spaced from the seam and a laterally reduced portion adjoining the seam, the second pocket portion having a laterally reduced portion adjacent the seam, one of the pocket portions of the one member having a projection extending from a base of the one pocket portion and in a direction toward the surface of the one member, a reinforcing element including laterally enlarged end portions connected to one another by a laterally reduced intermediate portion, one of the enlarged end portions of the element being received in the first pocket portion with the reduced intermediate portion of the element in the reduced pocket portion spanning the weld seam and a retainer engaging the projection to retain the element in the pocket spanning the weld seam.
In a further embodiment according to the present invention, there is provided a weld reinforcement comprising first and second members welded to one another forming an elongated weld seam, a continuous pocket formed in the members opening through corresponding surfaces thereof and defining first and second pocket portions on opposite sides of the weld seam, the first pocket portion having a laterally enlarged recess spaced from the seam and a reduced portion adjoining the seam, the second pocket portion having a laterally reduced portion adjacent the seam, a reinforcing element including laterally enlarged end portions connected to one another by a laterally reduced intermediate portion, one of the enlarged end portions of the element being received in the first pocket portion with the reduced intermediate portion of the element in the pocket spanning the weld seam and a cladding overlying the element and the first and second members along the surfaces thereof.
In a still further preferred embodiment according to the present invention, there is provided a weld reinforcement comprising first and second members welded to one another forming an elongated weld seam, a continuous, generally dogbone-shaped pocket formed in the members opening through corresponding surfaces thereof and defining first and second laterally enlarged recesses on opposite sides of the weld seam and joined to one another by a laterally reduced intermediate pocket section overlying the seam, a generally dogbone-shaped reinforcing element generally complementary in shape to the dogbone-shaped pocket and disposed in the pocket, the element including laterally enlarged end portions connected one to another by a laterally reduced intermediate section, the first and second pocket portions each having at least one edge extending generally parallel to the elongated weld seam and tapering away from the pocket portion in a direction from the base to the surface of the member, the enlarged end portions of the dogbone-shaped reinforcing element each having at least one edge extending generally parallel to the elongated weld seam and having a taper generally complementary to the taper of the associated pocket portion and means for retaining the dogbone-shaped reinforcing element in the pocket.